Screening methods for compounds that modulate the activity of g-protein coupled receptors

ABSTRACT

The present invention relates to a screening system for modulators of GPCRs. Further it relates to recombinant vector systems for the heterologous expression of heterodimeric G-protein coupled receptors (GPCRs) in eukaryotic host cells. Preferably the functional expression of engineered GPCRs for the perception of sweet and L-amino acid taste or more preferably the use of these receptors for the identification of functional ligands is also encompassed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 11/810,402, filed Jun. 5, 2007, which claims benefit of European Patent Application No. 06011710.8, filed Jun. 7, 2006, both of which are incorporated herein by reference in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a screening method for the identification of modulators (agonists as well as antagonists) of selected GPCRs and the thus identified modulators.

In a preferred embodiment of the present invention these modulators may be taste modulators. The present invention relates further to recombinant vector systems for the stable, heterologous expression of selected heterodimeric g-protein coupled receptors (GPCRs) in eucaryotic host cells. The functional expression of engineered GPCRs for the perception of sweet and L-amino acid taste and the use of said receptors for the identification of functional ligands is disclosed.

BACKGROUND OF THE INVENTION

In this specification, a number of documents are cited. The disclosure of these documents, including manufacturer's manuals and patent applications or patents, is herewith incorporated by reference in its entirety.

GPCRs represent the largest family of cell surface receptors with an estimated number of up to 1000 genes within the human genome characterized by a seven-transmembrane configuration as their main feature. (Bockaert and Pin, 1999; Pierce et al., 2002). GPCRs are activated by a multitude of different ligands, including peptides, proteins, lipids, small molecules, ions or even photons. Activated GPCRs alter their conformation allowing it to catalyze the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) on the α-subunit of a heterotrimeric g-protein coupled to the GPCR. Heterotrimeric G-proteins composed of one out of 18 different α-subunits, one out of 5 different β-subunits and one out of 11 different γ-subunits are usually classified by the nature of their α-subunit and generally grouped into four main classes: G_(s), which activates adenylyl cyclase; G_(i), which inhibits adenylyl cyclase; G_(q), which activates phospholipase C; and G_(12/13) with heterologous functions. In addition to the α-subunit dependent signaling the β/γ-subunits can function as signaling molecules on their own. GPCR dependent signaling becomes even more complex if it is considered that these receptors can exist as homo-oligomeric or hetero-oligomeric complexes. (George et al., 2002; Milligan et al., 2003; Salahpour et al., 2000). Hence, it is not surprising that GPCRs are responsible for the regulation of a wide variety of different physiological processes.

Recently the role of GPCRs in human senses like vision, olfaction and taste has been subject of intensified investigations. While the participation of the GPCR rhodopsin in visual sensing is one of the most comprehensively examined g-protein coupled receptor signaling examples of the last 30 years (Maeda et al., 2003), the role of GPCRs in olfaction and bitter taste as well as sweet taste was discovered in the 1990ies. (Buck and Axel, 1991; Firestein, 2001; Lindemann, 1996b; Lindemann, 2001).

The discovery of GPCR signaling in taste perception is closely connected to the discovery of tastant specific signaling in vertebrate taste cells. In electrophysical and biochemical studies it was apparent that tastant derived signaling resulted in typical GPCR dependent second messenger induction, e.g. cyclic nucleosides (cAMP, cGMP), inositol tri-phosphate (IP3) or calcium. (Kinnamon und Cummings, 1992; Kinnamon und Margolskee, 1996; Lindemann, 1996a). The participation of GPCRs in taste perception was further approved by the finding of the g-protein gustducin specifically expressed in vertebrate taste cells. (McLaughlin et al., 1992; Wong et al., 1996). On the other hand it was known from genetic mouse studies that the ability to sense sweet taste of e.g. saccharin was linked to the so called sac locus on mouse chromosome 4. (Bachmanov et al., 2001; Lush, 1989; Lush et al., 1995). Based on these data it was obvious to search for GPCR sequence tags in taste cell derived subtracted cDNA libraries or by performing genomic sequence scanning to further narrow down the mouse sac locus for the identification of GPCR analogs as putative taste receptors. These two approaches led to the rat, mouse and human receptor DNA sequences for the taste GPCRs T1R1 and T1R2 (Hoon et al., 1999; Hoon and Ryba, 1997) as well as T1R3. (Kitagawa et al., 2001; Li et al., 2001; Max et al., 2001; Montmayeur et al., 2001; Sainz et al., 2001). Homology alignments revealed that these taste receptors like the homodimeric metabotrophic glutamate receptor (mGluR), the heterodimeric γ-aminobutyric acid type B receptor (GABA_(B)R) and homodimeric extracellular calcium receptors are members of the small family of class C GPCRs. As a common characteristic most of the class C receptors exhibit a large extracellular amino terminal domain composed of a so called venus flytrap module (VFTM) and a cysteine rich domain (CRD) that connects the VFTM to the heptahelical domain. (Pin et al., 2003). Besides that homo- as well as hetero-oligomerisation was described for several of these class C receptors. (Bai et al., 1998; Kaupmann et al., 1998; Kunishima et al., 2000; White et al., 1998). Consequently, the characteristic feature of GPCR oligomerisation of class C receptors was tested for the putative sweet taste receptors T1R1, T1R2 and T1R3.

By recombinant heterologous expression in eucaryotic cell systems a functional expression and tastant specific activation of an artificially linked G-Protein dependent signaling cascade was demonstrated by calcium imaging. T1R receptors assemble to build up functional taste receptors. As a result of several investigations it was shown that the heterodimeric T1R1/T1R3 functions as a glutamate (umami) and L-amino acid receptor whereas the heterodimeric T1R2/T1R3 functions as a high affinity sugar and artificial sweetener receptor. Particularly, heterodimeric co-expression of T1R1 and T1R3 results in taste receptors that respond to umami taste and monosodium glutamate stimuli whereas heterodimeric co-expression of T1R2 and T1R3 results in taste receptors that respond to sweet stimuli like diverse sugars (e.g. glucose and sucrose), artificial sweetener (e.g. acesulfam K, cyclamat, saccharin) and sweet proteins like monellin, thaumatin, brazzein (Li et al., 2002; Nelson et al., 2002; Nelson et al., 2001; Zhao et al., 2002). A similar chronicle could be generated for the identification of GPCRs for the perception of bitter taste with the exception that so far no homo- or oligomerisation has been reported for these so called T2R-GPCRs. (Meyerhof et al., 2005).

The above discussed identification of genes coding for receptors responsible e.g. for taste perception, together with cloning said genes into appropriate vectors for the expression of said proteins in eukaryotic cells and the transformation of said cells with said vectors raised the expectation that screening systems and/or screening methods for GPCR modulators, i.e. agonists and antagonists of the above detailed receptors should be easy to be developed within a reasonable time.

This is reflected by a huge and still growing number of patent applications in this field.

The cloning of T1R1 is disclosed in different patent applications, e.g. in WO 03/025137; in WO 00/06952 (wherein it is designated GPCR-B3) US020040191862A1 and WO2005/033125.

The cloning of T1R2 is disclosed in patent applications WO 03/025137, US020040191862A1 and US020030040045A1

The cloning of T1R3 is disclosed in patent applications WO 03/025137, WO 03/025137, US020040191862A1 and US020030040045A1

A system for the expression of said proteins in eukaryotic cells is disclosed in patent applications WO 03/025137, WO 00/06952, US20040191862A1, WO2004069191 and US20030040045A1.

A screening system for putative taste modulators is disclosed in patent applications WO 00/06952, WO2004069191 and US20030040045A1.

Yet, nothing is to be told about the successful identification of new modulators, e.g. new artificial taste modulators such as new sweeteners utilizing such screening methods/systems.

The ongoing debate on obesity in developed countries and the growing health consciousness of consumers lead to an increasing demand of food and beverages with significant calorie reduction compared to products fully sweetened with carbohydrates such as sucrose, glucose, fructose or syrups such as HFCS 55 or 42. As the consumer usually is not willing to compromise on taste products should have similar sweetness intensity and taste quality as products regularly sweetened with these carbohydrates.

High intensity sweeteners are substances, which have no or virtually no calories and a sweetness potency several times higher than sugar. High intensity sweeteners or blends of high intensity sweeteners are used in food and beverages to achieve a sweet taste without adding calories to the products.

Most commonly used high intensity sweeteners are not from natural origin; They were discovered accidentally and are chemically synthesized. Most of them have a widespread approval in a large number of countries. Examples are substances such as acesulfame K, alitame, aspartame, cyclamate, neohesperidine dihydrochalcone, neotame, saccharin, and sucralose.

However, no high-intensity sweetener matches the taste profile of sugar completely. They differ in characteristics such as sweetness profile, side taste and off-taste characteristics. Proper blending of different high intensity sweeteners is known to overcome part of the taste limitations of single high-intensity sweeteners. But even if a more sugar-like sweetness profile is achieved in products with high-intensity sweeteners only, they still can be distinguished sensorically from their counterparts with just sugar or other carbohydrates by lack of mouthfeel and reduced flavour characteristics. Therefore a need exists for new high-intensity sweeteners which offer either alone or in blends with existing sweeteners sweetness profiles and flavour characteristics much closer to sugar than the existing products can offer.

Besides calorie reduction many of today's consumers are seeking for food and beverage products either without artificial additives or even being fully organic. Theoretically natural high-intensity sweeteners could fulfil this demand. A number of natural high-intensity sweeteners were discovered throughout past years such as stevioside, rebaudioside, brazzein, thaumatin, mogroside, glycyrrhizin, monatin, abrusoside, monellin, phyllodulcin and others. These are substances which naturally occur in different plants and can be obtained by selective extraction measures. Besides very limited approvals and in some cases difficulties to extract products on an industrial scale none of these products can claim to offer a sugar-like taste. In fact, all of these substances show a sweetness with a far slower onset than sucrose and a very lingering sweetness. Most of these products have strong side-taste and aftertaste characteristics such as bitter, mentholic or liquorice notes or show even strong cooling or numbing sensations. Therefore some of these products, e.g. thaumatin, can be rather regarded as being flavour enhancer than sweetener. Blending of two or more of these substances can not overcome these taste limitations. Therefore in the area of natural sweetener the need for new high-intensity sweeteners with a taste profile closer to sugar is even stronger than in the case of artificial sweeteners (O'Brien Nabors, 2001; Leatherhead Food R A, 2000; Grenby, 1996; von Rymon Lipinski und Schiweck, 1991). Therefore, there still exists a need in the art to identify and isolate new substances which may be used as modulators of taste perception, e.g. as sweeteners.

Notwithstanding the above, because of the high importance of these GPCRs in vivo, and the many different functions associated with said receptors, it has to be assumed that many of the modulators of GPCRs that might be identified by the method of the present invention may be of practical value.

Therefore, the availability of a simple and reliable screening system for modulators of said receptors would be of big importance.

In multicistronic expression vectors the coding sequences of different proteins are under the control of only one promoter and the different cistrons are connected via virus derived internal ribosomal entry sites (IRES) or cap independent translation enhancer (CITE). IRES or CITE elements confer a translation initiation independent from the otherwise necessary 5′-end of a messenger RNA, which is recognized by the eucaryotic ribosomes to start their scanning process for the first accessible translational start codon. (Fux et al., 2004; Hellen and Sarnow, 2001). So far multicistronic expression vectors have been described basically as dicistronic expression units for the coupled expression of a gene of interest linked via an cap-independent translation initiation site to a resistance marker (confering resistance to e.g. hygromycin, zeocin, neomycin) enabling selection of stable cell lines for heterologous mammalian expression studies. For this approach IRES or CITE dependent expression vectors are commercially available.

Reports on genuine multicistronic expression studies in mammalian systems with descriptions of tri-cistronic or even quadro-cistronic heterologous expression studies are rare and for the most part intended to improve inducible protein expression e.g. for gene therapy applications. However in this pioneering work multicistronic expressions have been mostly performed with small and soluble proteins like reporter genes (green fluorescent protein, yellow fluorescent protein, red fluorescent proteins, secreted alkaline phosphatase, secreted amylase) or engineered transactivators e.g. for macrolide- or streptogramin dependent expression or selection markers. Although these studies are aimed at potential therapeutic protein expression in gene therapy applications, only few genes with a therapeutic potential are mentioned within this studies (e.g. vascular endothelial growth factor (VEGF); the oncoprotein bcl-2). (Fussenegger et al., 1998; Hartenbach and Fussenegger, 2005; Kramer et al., 2003; Moser et al., 2000; Weber et al., 2005).

Concerning the expression of taste receptors there is one report on dicistronic expression of mouse taste receptors (mT2R8/5; mT1R3) each fused to green fluorescent protein and linked via an IRES element to red fluorescent protein. This approach was applied to trace and localize the expression pattern of taste receptors in neurons (Sugita and Shiba, 2005).

SUMMARY OF ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

Therefore, the technical problem underlying the present invention was to provide a cell based screening system for modulators of GPCRs, i.e. establishing a method which makes it possible to successfully identify important modulators of some selected GPCRs, preferably T1R-type taste GPCRs. It was a further problem underlying the present invention to provide means and methods for the isolation of identified modulators without undue burden.

The solution to the above mentioned technical problems is achieved by providing the embodiments characterized in the claims.

Therefore, specially preferred embodiments of the present invention are means and methods for the expression of heterodimeric T1R-type taste GPCRs, wherein the expression of these heterodimeric T1R-type taste GPCRs is effected by modulating the GPCR coding sequences as well as their expression in multicistronic operons.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts different GPCR nucleotide sequences of the TIR class, presented in an alignment analysis, with the underlying distance matrix shown below;

FIG. 2 depicts an exemplary multicistronic eukaryotic expression vector pTrix-Eb-R2R3;

FIG. 3 depicts a Ca^(Z+)-assay with HEK_Ga15#17R2R3b #8 measuring several sweet tastants;

FIG. 4 depicts an alternative exemplary multicistronic eukaryotic expression vector p Trix-Eb-R2R3.

DETAILED DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

The above problem can surprisingly yet easily be solved by providing a method for the identification of modulators of GPCRs, comprising the steps of:

-   -   a. transforming eucaryotic host cells with genetic sequences         coding for one or more than one GPCR(s),     -   b. cultivation of the transformed host cells under conditions         sufficient to ensure the functional expression of said one or         more GPCRs,     -   c. contacting the cultivated host cell expressing the one or         more GPCRs in a functional manner with a potential modulator of         the selected one or more GPCRs,     -   d. measuring a specific cellular response on exposure of the         transformed host cell to the potential modulator, and     -   e. selecting an identificate, which induces a specific response.

Within the context of the present invention the term “modulator of GPCRs” is intended to mean a substance, which if applied in vivo, would (a) trigger an intracellular response by binding to the selected GPCR or (b) would inhibit such a response in the presence of a sweet compound. E.g. in the case of sweet receptors as the selected GPCR this modulator could be a substance which tastes sweet or which potentiates the sweetness of a sweet tasting compound without being sweet on its own or which inhibits the sweetness of sweet substances.

The term “conditions sufficient to ensure the functional expression” shall be understood as a summary of all conditions needed to ensure the functional expression of the GPCRs, rather than cell culture conditions. In a preferred embodiment of the present invention this term pertains to the vector used for transformation being a tricistronic vector. Further, this expression encompasses culturing cells which comprise a functional g-.protein. Under any circumstances, the functional expression of the GPCR(s) must be ensured.

The terms “functional expression” or “expressing in a functional manner” means within the context of the present invention that the selected receptor maintains its ability to interact specifically with the same substances the receptor would interact with in the in vivo situation.

By “selected cellular response” e.g. changes in intracellular calcium levels have to be understood, which can be measured easily e.g. by using dyes like Calcium 3, Fura-2, Fluo-4, Indo-1 or the calcium dependent reporter protein aequorin. Any cellular response related to the activity of the selected GPCRs is encompassed. Further examples of measuring the activity of GPCRs are (i) the activation of the second messenger cyclic adenosine monophosphate (cAMP) which can be quantified in GPCR dependent cell based assays with luminescent tags or with cAMP dependent reporter gene expression (e.g. Luciferase, SEAP or similar) or with the melanophore technology using frog skin cells; (ii) the measurement of β-arrestin binding to the targeted GPCR after ligand induced activation of the GPCR via bioluminescence resonance energy transfer (BRET) measurements or GFP-tagged β-arrestin movement imaging; (iii) measuring the agonist occupation of a GPCR and respectively the activation of the associated G-Protein, which can be quantified by using the radioactive, non-hydrolyzable analogue of GTP, [35S]GTPγS; (IV) the use of response element controlled reporter gene assays for the readout of GPCR activated interconnected pathways, including those involving MAP kinases, nonreceptor tyrosine kinases, receptor tyrosine kinases, phosphatidylinositol 3-kinases, and JNKs. (Eglen, 2005; Filmore, 2004; Milligan, 2003).

The term “identificate” is directed to a potential modulator after being identified as a true modulator of GPCR or the GPCR specific signaling cascade.

In a preferred embodiment the method is characterized in that the host cell is selected from the group consisting of: HEK293 (human embryo kidney), Hela (Human Negroid cervix epitheloid carcinoma), HT29 (Human Caucasian colon adenocarcinoma grade II), A431 (human squamous carcinoma), IMR 32 (human caucasian neuroblastoma), K562 (Human Caucasian chronic myelogenous leukaemia), U937 (Human Caucasian histiocytic lymphoma), MDA-MB-231 (Human Caucasian breast adenocarcinoma), SK− N-BE(2) (Human Caucasian neuroblastoma), SH-SY5Y (Human neuroblastoma), HL60 (human promyelocytic leukemia) or eukaryotic non-human cell lines like CHO-K1 (Hamster Chinese ovary), COS-7 (Monkey African green kidney, SV40 transformed), S49 (mouse lymphoma), Ltk (Mouse C34/an connective tissue), NG108-15 (Mouse neuroblastoma×Rat glioma hybrid), B50 (Rat nervous tissue neuronal, ECACC), 06 (Rat glial tumour), Jurkat (Human leukaemic T cell lymphoblast), BHK (Hamster Syrian kidney), Neuro-2a (Mouse Albino neuroblastoma), NIH/3T3 (mouse embryo fibroblast), preferably HEK293 (human embryo kidney), Hela (Human Negroid cervix epitheloid carcinoma), CHO-K1 (Hamster Chinese ovary) or Neuro-2a (Mouse Albino neuroblastoma).

It is essential that the host cell expresses a functional g-protein, preferably G-alpha15, either naturally or by means of genetic alteration of the host cell. Means for genetic alteration of eucaryotic cells are well known in the art, and need not to be discussed in depth here. The DNA-Sequences of G-proteins, e.g. G-alpha15 have already been described in the art.

A further preferred embodiment of the method of the present invention is characterized in that the one or more GPCR(s) is (are) selected from the group consisting of T1R or T2R taste receptors, preferably a T1R-type GPCR, especially T1R1, T1R2 and/or T1R3.

A further preferred embodiment of the method of the present invention is characterized in that two or more GPCRs are expressed in a heterologous co-expression of at least two different T1R-type GPCRs, preferably T1R1/T1R3, more preferably T1R2/T1R3.

A further preferred embodiment of the method of the present invention is characterized in that the transformation is accomplished with a multicistronic vector, preferably a tricistronic vector.

The vector of the present invention is an expression vector. By the term “expression vector” it is meant that the vector is used to transform a selected eucaryotic host cell, which, after transformation, expresses the gene or genes encoded by said vector. Expression vectors may be e.g. cloning vectors, binary vectors or integrating vector. Expression comprises transcription of the encoded nucleic acids into a functional (translatable) mRNA. Therefore, the respective control elements should be present, e.g. a sequence promoting transcription of the messenger (a promoter) and (optionally) a polyadenylation signal. Means for the expression of heterologous genes in eukaryotic cells are very well studied.

For the propagation of such vectors, usually prokaryotic cells such as E. coli are used. Therefore, although the vectors of the present invention are designed to work as expression vectors in eukaryotic cells, they also carry elements for propagation in prokaryotic cells, e.g. an origin of replication (ori) and an antibiotics resistance gene, e.g. amp^(r), kan^(r) and similar. Means for the propagation of vectors in prokaryotes are well known in the art.

A list of possible eukaryotic expression vectors usable in the present invention, optionally usable for propagation in a prokaryotic host, and already commercially available comprises: pCR1000, pCDM8, pcDNA1, pcDNA1.1, pcDNA1/Amp, pcDNA1.1/Amp and pcDNA1/Neo, pcDNA3, pcDNA3.1, pcDNA3.2, pcDNA6.2, pDEST26, pDEST27, pCR3.1, pcDNA3.1 His, pDisplay (Invitrogen); pTriEx (pTriEx-2-Hygro) (Novagen); pSI, pCI (pCI-neo), pTargeT (Promega); pERV3, pFB-ERV, pCFB-EGSH, pDual, pCMV-Script (Stratagene); pNEBR (New England Biolabs), pEAK (Edge Biosystems).

A further preferred embodiment of the method of the present invention is characterized in that the multicistronic vector comprises a multicistronic expression unit comprising downstream from a promoter for the expression in an eucaryotic host and functionally linked thereto, the following cistrons:

-   -   a. GPCR₁     -   b. GPCR₂ and     -   c. a selection marker,         wherein the promoter preferably is a strong promoter suitable         for use in the selected host cell, more preferably being         selected from the group consisting of cytomegalovirus promoter         (P-CMV), human elongation factor 1 alpha promoter (P-E1α), human         ubiquitin promoter (P-ubi), simian virus promoter (P-SV40), Rous         sarcoma virus long terminal repeat promoter (P-RSV-LTR) and         similar, wherein the GPCR₁ and the GPCR₂ are independently from         another being selected from the group consisting of T1R or T2R         taste receptors, preferably of the group of T1R receptors, more         preferably a combination of T1R1-T1R3 or T1R2-T1R3, wherein the         selection marker is being selected from the group consisting of         hygromycin^(r), zeocin^(r), neomycin^(r), blasticidin^(r) or         puromycin^(r), and wherein both the GPCR1 and the GPCR2 as well         as the selection marker are functionally connected by         intervening IRES selected from the group consisting of         IRES_(EMCV), derived from encephalomyocarditis virus (synonym:         CITE_(EMCV)); IRES_(GTX), derived from the GTX homeodomain mRNA;         IRES_(Rbm3), derived from cold-inducible Rbm3; IRES_(PV),         derived of polioviral origin, IRES_(RV), derived from         rhinovirus, IRES_(FMDV), derived from food and mouth disease         virus; IRE_(HV), derived from hepatitis C virus, IRES_(CSFV),         derived from classic swine fever virus, IRES_(BVPV), derived         from bovine viral diarrhea virus; IRES_(FMLV), derived from         friend murine leukemia virus gag mRNA; IRES_(MMLV), derived from         moloney murine leukemia virus gag mRNA; IRES_(HIV), derived from         human immunodefiency virus env mRNA; IRES_(PSIV), derived from         Plautia stall intestine virus; IRES_(RPV), derived from         Rhopalosiphum padi virus; IRES_(KSH), derived from Karposi's         sarcoma-associated herpesvirus, preferably the IRES_(EMCV) being         derived from encephalomyocarditis virus (synonym: CITE_(EMCV))         and wherein the multicistronic expression unit is terminated by         a polyadenylation signal.

The term “functionally linked thereto” means within the context of the present invention that the components described are linked together to function in their intended manner.

A further preferred embodiment of the method of the present invention is characterized in that the multicistronic vector additionally comprises a genetic sequence coding for a g-protein or an equivalent thereof, preferably G-alpha 15 or an equivalent thereof, which is coded as an monocistronic unit consisting of a promoter, the gene of the selected g-protein and a polyadenylation site.

A further preferred embodiment of the method of the present invention is characterized in that the multicistronic vector additionally comprises a genetic sequence coding for a g-protein or an equivalent thereof, preferably G-alpha 15 or an equivalent thereof, which is coded as a fourth cistron in a quadrocistronic arrangement via an additional IRES element.

A further preferred embodiment of the method of the present invention is characterized in that the multicistronic vector additionally comprises a genetic sequence coding for a g-protein or an equivalent thereof, preferably G-alpha 15 or an equivalent thereof, the g-protein preferably being fused in-frame to above GPCR₁ and/or GPCR₂.

According to the present invention G proteins such as G-alpha15 or G-alpha16 or other promiscuous G proteins or G protein variants, or an endogenous G protein like gustducin, or another g-protein that when expressed in association with the multicistronically encoded GPCR(s) produces a functional read out may be used. In addition, G-beta and G-gamma proteins may also be used.

Subvariants of G-alpha 15 and/or G-alpha 16 with modified N-termini are also well known in the art, and can be used accordingly.

Essentially any chemical compound can be employed as a potential modulator or ligand in the assays according to the present invention. Compounds tested as G-protein coupled receptor modulators can be any small chemical compound, or biological entity (e.g., protein, sugar, nucleic acid, lipid). Test compounds will typically be small chemical molecules and peptides. Generally, the compounds used as potential modulators can be dissolved in aqueous or organic (e.g., DMSO-based) solutions. The assays are designed to screen large chemical libraries by automating the assay steps and providing compounds from any convenient source. Assays are typically run in parallel, for example, in microtiter formats on microtiter plates in robotic assays. There are many suppliers of chemical compounds, including Sigma (St. Louis, Mo.), Aldrich (St. Louis, Mo.), Sigma-Aldrich (St. Louis, Mo.), Fluka Chemika-Biochemica Analytika (Buchs, Switzerland), for example. Also, compounds may be synthesized by methods known in the art.

So-called high throughput screening methods typically involve providing a combinatorial chemical or peptide library containing a large number of potential therapeutic compounds (e.g., ligand or modulator compounds). Such combinatorial chemical libraries or ligand libraries are then screened in one or more assays to identify those library members (e.g., particular chemical species or subclasses) that display a desired characteristic activity. The compounds so identified can serve as conventional lead compounds, or can themselves be used as potential or actual therapeutics.

A combinatorial chemical library is a collection of diverse chemical compounds generated either by chemical synthesis or biological synthesis, by combining a number of chemical building blocks (i.e., reagents such as amino acids). As an example, a linear combinatorial library, e.g., a polypeptide or peptide library, is formed by combining a set of chemical building blocks in every possible way for a given compound length (i.e., the number of amino acids in a polypeptide or peptide compound). Millions of chemical compounds can be synthesized through such combinatorial mixing of chemical building blocks.

The preparation and screening of combinatorial chemical libraries is well known to those having skill in the pertinent art. Combinatorial libraries include, without limitation, peptide libraries (e.g. U.S. Pat. No. 5,010,175; Furka, 1991, Int. J. Pept. Prot. Res., 37:487-493; and Houghton et al., 1991, Nature, 354:84-88). Other chemistries for generating chemical diversity libraries can also be used. Nonlimiting examples of chemical diversity library chemistries include, peptides (PCT Publication No. WO 91/019735), encoded peptides (PCT Publication No. WO 93/20242), random bio-oligomers (PCT Publication No. WO 92/00091), benzodiazepines (U.S. Pat. No. 5,288,514), diversomers such as hydantoins, benzodiazepines and dipeptides (Hobbs et al., 1993, Proc. Natl. Acad. Sci. USA, 90:6909-6913), vinylogous polypeptides (Hagihara et al., 1992, J. Amer. Chem. Soc., 114:6568), nonpeptidal peptidomimetics with glucose scaffolding (Hirschmann et al., 1992, J. Amer. Chem. Soc., 114:9217-9218), analogous organic synthesis of small compound libraries (Chen et al., 1994, J. Amer. Chem. Soc., 116:2661), oligocarbamates (Cho et al., 1993, Science, 261:1303), and/or peptidyl phosphonates (Campbell et al., 1994, J. Org. Chem., 59:658), nucleic acid libraries (see Ausubel, Berger and Sambrook, all supra), peptide nucleic acid libraries (U.S. Pat. No. 5,539,083), antibody libraries (e.g., Vaughn et al., 1996, Nature Biotechnology, 14(3):309-314) and PCT/US96/10287), carbohydrate libraries (e.g., Liang et al., 1996, Science, 274-1520-1522) and U.S. Pat. No. 5,593,853), small organic molecule libraries (e.g., benzodiazepines, Baum C&EN, Jan. 18, 1993, page 33; and U.S. Pat. No. 5,288,514; isoprenoids, U.S. Pat. No. 5,569,588; thiazolidinones and metathiazanones, U.S. Pat. No. 5,549,974; pyrrolidines, U.S. Pat. Nos. 5,525,735 and 5,519,134; morpholino compounds, U.S. Pat. No. 5,506,337; and the like).

Devices for the preparation of combinatorial libraries are commercially available (e.g., 357 MPS, 390 MPS, Advanced Chem Tech, Louisville Ky.; Symphony, Rainin, Woburn, Mass.; 433A Applied Biosystems, Foster City, Calif.; 9050 Plus, Millipore, Bedford, Mass.). In addition, a large number of combinatorial libraries are commercially available (e.g., ComGenex, Princeton, N.J.; Asinex, Moscow, Russia; Tripos, Inc., St. Louis, Mo.; ChemStar, Ltd., Moscow, Russia; 3D Pharmaceuticals, Exton, Pa.; Martek Biosciences, Columbia, Md., and the like).

In one embodiment, the invention provides solid phase based in vitro assays in a high throughput format, where the cell or tissue expressing an ion channel is attached to a solid phase substrate. In such high throughput assays, it is possible to screen up to several thousand different modulators or ligands in a single day. In particular, each well of a microtiter plate can be used to perform a separate assay against a selected potential modulator, or, if concentration or incubation time effects are to be observed, every 5-10 wells can test a single modulator. Thus, a single standard microtiter plate can assay about 96 modulators. If 1536 well plates are used, then a single plate can easily assay from about 100 to about 1500 different compounds. It is possible to assay several different plates per day; thus, for example, assay screens for up to about 6,000-20,000 different compounds are possible using the described integrated systems.

In another aspect, the present invention encompasses screening and small molecule (e.g., drug) detection assays which involve the detection or identification of small molecules that can bind to a given protein, i.e., a taste receptor polypeptide or peptide. Particularly preferred are assays suitable for high throughput screening methodologies.

In such binding-based detection, identification, or screening assays, a functional assay is not typically required. All that is needed is a target protein, preferably substantially purified, and a library or panel of compounds (e.g., ligands, drugs, small molecules) or biological entities to be screened or assayed for binding to the protein target. Preferably, most small molecules that bind to the target protein will modulate activity in some manner, due to preferential, higher affinity binding to functional areas or sites on the protein.

An example of such an assay is the fluorescence based thermal shift assay (3-Dimensional Pharmaceuticals, Inc., 3DP, Exton, Pa.) as described in U.S. Pat. Nos. 6,020,141 and 6,036,920 to Pantoliano et al.; see also, J. Zimmerman, 2000, Gen. Eng. News, 20(8)). The assay allows the detection of small molecules (e.g., drugs, ligands) that bind to expressed, and preferably purified, ion channel polypeptide based on affinity of binding determinations by analyzing thermal unfolding curves of protein-drug or ligand complexes. The drugs or binding molecules determined by this technique can be further assayed, if desired, by methods, such as those described herein, to determine if the molecules affect or modulate function or activity of the target protein.

Compounds which are identified according to the methods provided herein, and which modulate or regulate the biological activity or physiology of the T1R polypeptides according to the present invention are a preferred embodiment of this invention. It is contemplated that such modulatory compounds may be employed as outlined above.

Assays that may be utilized with one or more T1Rs according to the invention include by way of example assays that utilize a genetic selection for living cells; assays that utilize whole cells or membrane fragments or purified taste receptor proteins; assays that utilize second messengers such as cAMP and IP3, assays that detect the translocation of arrestin to the cell surface, assays that detect the loss of receptor expression on the cell surface (internalization) by tested ligands, direct ligand-binding assays, competitive-binding assays with inhibitors, assays using in vitro translated protein, assays that detect conformational changes upon the binding of a ligand (e.g., as evidenced by proteolysis, fluorescence, or NMR), behavioral assays that utilize transgenic non-human animals that express a taste GPCR or a combination thereof, such as flies, worms, or mice, assays that utilize cells infected with recombinant viruses that contain taste GPCR genes, preferably measuring the change of intracellular calcium levels relative to intracellular calcium levels without contacting the cell to the modulator.

A further preferred embodiment of the method of the present invention is characterized in that the change is an increase.

A further preferred embodiment of the method of the present invention is characterized in that the change is an increase relative to intracellular calcium levels when the cell is contacted by a modulator in the peresence of a sweet compound thereby enhancing the calcium level above the level of the level generated by the sweet compound alone. In this case the modulator can either be sweet on its own or either be a tasteless compound without having an bioactive potential to activate T1Rs or combinations thereof on its own. The sweet compound preferably being selected from the group consisting of glucose, fructose, saccharose, acesulfam K, saccharin, cyclamat, aspartam, xylitol, stevioside, sucralose, thaumatin, monellin, brazzein, perillartine, glycyrrhizin, sucronic acid, P-4000, SC45647, NC174, neohesperidin and sweet tasting amino acids, more preferably selected from the group consisting of glucose, fructose, saccharose or xylitol.

A further preferred embodiment of the method of the present invention is characterized in that the change is a decrease relative to intracellular calcium levels when the cell is contacted by a sweet compound instead of the modulator, the sweet compound preferably being selected from the non-exclusive group consisting of glucose, fructose, saccharose, acesulfam K, saccharin, cyclamat, aspartam, xylitol, stevioside, sucralose, thaumatin, monellin, brazzein, perillartine, glycyrrhizin, sucronic acid, P-4000, SC45647, NC174, neohesperidin and sweet tasting amino acids.

A still further preferred embodiment of the present invention are multiparameter optimized nucleic acid molecules coding for a GPCR, preferably a T1R-type GPCR, more preferably a T1R1, T1R2 or T1R3 or a functionally equivalent receptor protein, and even more preferably coding for proteins consisting of the amino acids sequences according to SEQ ID NO: 1 and/or SEQ ID NO: 2 or, optionally, functionally equivalent proteins.

These nucleic acid molecules in a preferred embodiment have the sequence as depicted in SEQ ID NOs 3 and 4.

Nucleic acid molecules which are functionally equivalent to the molecules as depicted in SEQ ID NOs 3 and 4 are also encompassed by the present invention.

Within the context of the present invention, functionally equivalent proteins have the same or a very similar function in-vivo. Preferably, functionally equivalent proteins share at least 60%, more preferably at least 80%, especially at least 90%, advantageously at least 99% identity in their amino acid sequence.

A functionally equivalent nucleic acid codes for a functionally equivalent protein and it is optimized for expression in a eucaryotic cell. According to a multiparameter optimization the codon usage was adapted to the codon bias of Homo sapiens genes. In addition, regions of very high (>80%) or very low (<30%) GC content have been avoided where possible. During the optimization process following cis-acting sequence motifs were avoided:

a) internal TATA-boxes, chi-sites and ribosomal entry sites b) AT-rich or GC-rich sequence stretches c) ARE, INS, CRS sequence elements d) repeat sequences and RNA secondary structures, as well as e) (cryptic) splice donor and acceptor sites, branch points.

The optimization process, starting from the human wild type sequences (wt_hT1R) revealed several sequences; surprisingly the sequences which offered the best performance (sh_T1R) in generating functional stable cell lines on the basis of the multicistronic expression approach were not those with the theoretically optimal sequence (opt_hT1R) nor the wild type sequences. The differences of the here exemplified sequences are illustrated as a phylogenetic alignment (generated by the bioinformatic software clustal X) with an accompanying relational distance matrix in FIG. 1.

A very preferred embodiment of the present invention are multicistronic expression vectors comprising more than one cistron coding for a GPCR.

The said multicistronic expression vector preferably comprises downstream from a promoter for the expression in an eucaryotic host cell and functionally linked thereto, the following cistrons:

-   -   a. GPCR₁     -   b. GPCR₂ and a     -   c. selection marker,         wherein the promoter preferably is a strong promoter, more         preferably being selected from the group consisting of         cytomegalovirus promoter (P-CMV), human elongation factor 1         alpha promoter (P-E1α), human ubiquitin promoter (P-ubi), simian         virus promoter (P-SV40), Rous sarcoma virus long terminal repeat         promoter (P-RSV-LTR) and similar wherein the GPCR₁ and the GPCR₂         are independently from another being selected from the group         consisting of T1R or T2R taste receptors, preferably of the         group of T1R receptors, more preferably a combination of         T1R1-T1R3 or T1R2-T1R3, and wherein the selection marker is         being selected from the group consisting of hygromycin^(r),         zeocin^(r), neomycin^(r), blasticidin^(r) or puromycin^(r), and         wherein both the GPCR1 and the GPCR2 as well as the selection         marker are functionally connected by intervening IRES selected         from the group consisting of IRES_(EMCV), derived from         encephalomyocarditis virus (synonym: CITE_(EMCV)); IRES_(GTX),         derived from the GTX homeodomain mRNA; IRES_(Rbm3), derived from         cold-inducible Rbm3; IRES_(PV), derived of polioviral origin,         IRES_(RV), derived from rhinovirus, IRES_(FMDV), derived from         food and mouth disease virus; IRE_(HV), derived from hepatitis C         virus, IRES_(CSFV), derived from classic swine fever virus,         IRES_(BVDV), derived from bovine viral diarrhea virus;         IRES_(FMLV), derived from friend murine leukemia virus gag mRNA;         IRES_(MMLV), derived from moloney murine leukemia virus gag         mRNA; IRES_(HIV), derived from human immunodefiency virus env         mRNA; IRES_(PSIV), derived from Plautia stall intestine virus;         IRES_(RPV), derived from Rhopalosiphum padi virus; IRES_(KSH),         derived from Karposi's sarcoma-associated herpesvirus,         preferably., the IRES_(EMCV), derived from encephalomyocarditis         virus (synonym: CITE_(EMCV)) and wherein the multicistronic         expression unit is terminated by a polyadenylation signal.

Another preferred embodiment of the present invention is a multicistronic vector as defined above, the vector additionally comprising a cistron coding for a G-protein, preferably G-alpha 15, the g-protein preferably being located between the last GPCR and the selection marker and being functionally connected to both of them via an IRES element as defined supra.

A further preferred embodiment are cell lines transformed with the vectors of the present invention. Respective eukaryotic cell lines are amphibian, worm, insect or mammalian cells such as CHO, Hela, Hek-293 and the like, e.g., cultured cells, explants, and cells in vivo.

Preferably, the host cell is selected from the group consisting of: HEK293 (human embryo kidney), Hela (Human Negroid cervix epitheloid carcinoma), HT29 (Human Caucasian colon adenocarcinoma grade II), A431 (human squamous carcinoma), IMR 32 (human caucasian neuroblastoma), K562 (Human Caucasian chronic myelogenous leukaemia), U937 (Human Caucasian histiocytic lymphoma), MDA-MB-231 (Human Caucasian breast adenocarcinoma), SK-N-BE(2) (Human Caucasian neuroblastoma), SH-SY5Y (Human neuroblastoma), HL60 (human promyelocytic leukemia) or eukaryotic non-human cell lines like CHO-K1 (Hamster Chinese ovary), COS-7 (Monkey African green kidney, SV40 transformed), S49 (mouse lymphoma), Ltk (Mouse C34/An connective tissue), NG108-15 (Mouse neuroblastoma×Rat glioma hybrid), B50 (Rat nervous tissue neuronal, ECACC), C6 (Rat glial tumour), Jurkat (Human leukaemic T cell lymphoblast), BHK (Hamster Syrian kidney), Neuro-2a (Mouse Albino neuroblastoma), NIH/3T3 (mouse embryo fibroblast), preferably HEK293 (human embryo kidney), Hela (Human Negroid cervix epitheloid carcinoma), CHO-K1 (Hamster Chinese ovary) or Neuro-2a (Mouse Albino neuroblastoma).

Further, preferred embodiments of the present invention are the modulators of GPCRs identifiable by the methods of the present invention and further characterized by being selected from the group consisting of sugars, steroids, tannins and lignans, terpenes, quinons, macrocycles, heterocycles, N-heterocycles and O-heterocycles, aliphatics and polyketides, flavonoids, proteins, peptides and amino acids, alkaloids and arenes, as stated above.

The intense investigations of the present inventors that led to the establishment of the method of the present invention showed that the recombinant co-expression of said taste receptors as stable integrants of eucaryotic host cells is hampered by a common instability. Although the phenomenon is far away from being understood, it might be possible that the expression and specific signal transduction of these T1R-type receptors interferes in a negative fashion with the cell physiology of such stable host cells.

Within the context of the present invention, an instable integration is characterized by a rapid loss of functionality within the first 10 passages of a selected recombinant cell line clone. Within these first ten passages it becomes obvious that more and more cells of a selected recombinant clone loose their characteristic functional taste GPCR expression, indicated by passage dependent incremental decrease of sweetener induced calcium inductions (Fluo-4 measurements). By way of contrast, the stable integrants of the present invention are characterized in that they show a functional expression of said taste receptors at least over 50 passages, indicated by stable sweetener induced calcium inductions (Fluo-4 measurements) with a selected taste receptors expressing cell line clone.

As becomes clear from the present specification of the invention, the use and the hetero-oligomeric expression of the taste receptor combinations T1R1/T1R3 and more preferred T1R2/T1R3 to identify compounds as modulators of sweet taste in the field of tastants are especially preferred embodiments of the present invention.

Accordingly, this invention relates to recombinantly engineered T1R-type GPCRs having activity in cell based tastant assays, obtainable by (a) improving the coding sequence of the human sweet taste receptors (T1Rs) and/or (b) cloning them in a multicistronic expression unit for coordinated expression and simultaneous selection for the generation of stable eucaryotic cell systems; and/or (c) tagging one or both GPCRs in the hetero-dimeric expression complex with G-proteins or G-Protein chimeras to enable an improved fluorescence read out for the identification of GPCR modulators, preferably sweet taste modulating compounds.

The receptor sequences applied for the here presented invention are furthermore exemplified in Example 1. By adding a prefix the optimized receptors have been termed shT1R2 and shT1R3. The respective amino acid sequences are depicted in SEQ ID NOs 1 and 2, the respective nucleic acid sequences in SEQ ID NOs 3 and 4.

A further means of the present invention to overcome the instability of co-expressed GPCRs, preferably taste receptors T1R2 and T1R3 in stable cell line development is the construction of multicistronic transcription units. The method for obtaining a multicistronic expression vector for the simultaneous expression of taste receptors and a selection marker of the invention is further exemplified in Example 2. As stated above the development of T1R2/T1R3 expressing stable cell lines e.g. Hek293, CHO, Hela for their use in cell based assay systems is hampered by upcoming instabilities within the ongoing eucaryotic cell cultivation process and passaging numbers. For the generation of stable cell lines in the art most often expression vectors are used were the selection marker, conferring resistance to e.g. neomycin, hygromycin, zeocin, blasticidin or puromycin, is in fact coded on the same plasmid vector under the control of a independent promoter.

This situation can be avoided if the receptors and the selection marker are encoded in a multicistronic transcription unit.

However, so far multicistronic expression vectors have been described only for the use of inducible protein expression e.g. for gene therapy applications (Fussenegger et al., 1998; Moser et al., 2000).

For the present invention the two taste receptors shT1R2 and shT1R3 have been cloned in the first and second position of a tricistronic expression unit, whereas the deaminase gene conferring resistance to the selection marker blasticidin was cloned into the third position of the expression unit. The tricistronic unit is under the control of the strong human elongation factor 1 alpha promoter and the genes in the second and third position are preceded by an IRES element to facilitate their translation initiation in the resulting mRNA.

So far an eight kb multicistronic expression system for the stable functional expression of membrane located GPCRs has not been described. Usually these multicistronic systems—already in expressions of much smaller gene products like fluorescent proteins—are characterized by a non-stoichiometric and decreasing expression with increased distance from the promoter. Since the taste GPCRs significantly exceed the size of GFP or genes so far used in tri- or quadrocistronic expression systems; and the fact that the GPCRs have to be co-expressed to form a heterodimeric protein complex to function as sweetener responsive receptors it was an unforeseeable and surprising result of the intense investigations leading to the present invention that only the use of multicistronic vectors, i.e. tri- or tetracistronic vectors let to satisfying results in establishing the screening method of the present invention.

In a further embodiment, this invention relates to methods using the before mentioned multicistronic expression vector to create stable cell lines suited to express T1R polypeptides to construct a cell based screening tool for the search of novel compounds in the field of sweet taste modulators. Preferably, the cells comprise a functional G protein, e.g., G-alpha-15, G-alpha-16 or chimeric G protein like the ones with altered c-terminus previously identified:

a) substitution of the last five amino acids of G-alpha 15:

-   -   G-alpha_(—)15 (EINLL), replaced with EYNLV (G-alpha q and         G-alpha 11), EFNLV (G-alpha 14), QYELL (G-alpha s and G-alpha         olf), DCGLF (G-alpha i1, G-alpha i2, G-alpha t1, G-alpha t2, and         G-alpha gust), ECGLY (G-alpha i3), GCGLY (G-alpha o1 and G-alpha         o2), YIGLC (G-alpha z), DIMLQ (G-alpha 12), QLMLQ (G-alpha 13)         or G-alpha 16 replaced with ECGLY (G-alpha 16 i3)         b) substitution of the last 44 carboxy-terminal amino acids of         G-alpha-16:     -   G-alpha 16gust44 (44 amino acids of the G-protein gustducin)     -   G-alpha 16z44 (44 amino acids of G-alpha-z);     -   G-alpha 16C44i2 (44 amino acids of the G-alpha-i2);     -   G-alpha 16C44i3 (44 amino acids of the G-alpha-i3);         or another G protein that is capable of coupling the chimeric         receptor to an intracellular signaling pathway or to a signaling         protein such as phospholipase C. (Li et al., 2002; Offermanns,         2003; Offermanns and Simon, 1995; Ueda et al., 2003). For this         purpose HEK 293 cells comprising a functional G protein have         been transfected with the vector pTrix-Eb_R2R3, cultured in the         presence of blasticidine, and stable cell lines have been         selected. As shown in Example 3 such cells have been found to         exhibit responses to taste stimuli and in addition these cell         lines have—compared to those derived with monocistronic T1R         expression vectors—a pronounced stable expression of the T1R         polypeptides. A stable cell line based on the expression vector         pTrix-Eb_R2R3 (see example 3) shows calcium responses to several         artificial sweeteners and a natural sweet protein. These         responses can still be measured after more than 50 passages of         the clone under selection with blasticidin and G418. Such         results could not be achieved with monocistronic or bicistronic         expressions. RT-PCR results of monocistronic clones revealed the         loss of both receptors with the initial 5 to 10 passages under         selection conditions. The stable cell line HEK_Ga15#17R2R3b #8         also shows dosis-dependent calcium responses for the above shown         sweet tastants which correlated with physiological taste         responses.

A further unexpected effect is that these multicistronic stable taste receptor expressing cell lines are well suited for natural compound screening using complex microbial extract preparations. For the isolation and preparation of such complex screening samples standard microbial methods can be applied which encompass the cultivation of microorganisms. Bioactive metabolites of interest from such cultivations may include intracellular molecules as well as those secreted into the cultivation media. Depending on the number of samples to be processed, a choice can generally be made from different options: (i) heating of the whole broth; (ii) filtration of broth supernatant through a high molecular weight exclusion filter with subsequent freeze-drying of the filtrate; (iii) extraction of eihter the whole broth (ultrasonication, french press, cell disruption bombs or similar devices) or the broth supernatant with organic solvents of varying polarity (e.g. chloroform, aceton, methanol or ethyl acetat) followed by evaporation of the extracts to dryness. (iv) Mixing the whole broth or the broth supernatant with resins (e.g. AMBERLITE®, XAD-2, XAD-4, XAD-9 or similar) with subsequent solid phase extraction with organic solvents (e.g. acetone, acetic acid, butanol, ethanol, ethyl acetate, methanol, polyglycols or similar) either fractionated by applying increasing solvent concentrations or as whole batch by applying the pure solvent without fractionation. Solid phase extracts can be used directly in suited assay systems or further evaporated to dryness and resolved under standardized conditions for subsequent testing, e.g. the activation of taste receptors in cell based assays. Activation of T1R receptors in such cells can be detected using any standard methods, such as by detecting changes in intracellular calcium by detecting Fluo-4 dependent fluorescence in the cell. Such an assay is the basis of the experimental findings presented in this application.

In a similar manner plant extracts well known to the skilled man may be used as well.

In FIG. 3 the activity of a stable cell line derived with the multicistronic expression plasmid pTrix-Eb-R2R3 in a G-alpha 15 G-Protein background is depicted. The data of FIG. 3 document that these cell line amongst several other isolated cell lines, shows T1R2/T1R3 dependent activity in cell based assays towards several tastants like cyclamate, D-phenylalanine, saccharin, acesulfam K, aspartam and thaumatin.

In a further embodiment, this invention relates to the use of T1R-G-protein fusion genes for the development of T1R-dependent cell based assays. It has been shown that GPCRs can be fused in frame with different polypeptides without loosing their functional activity (Milligan et al., 2003). From the present data it cannot be foreseen that a fusion of a functional G-Protein to any GPCR leads to an improved activity compared to the wild type unfused situation. However it is scheduled to develop T1R2- and/or T1R3-fusion proteins in which the T1R-GPCR is fused to a functional G-Protein which is able to connect the T1R dependent signaling to the calcium inducing pathways used for T1R dependent activity measurement in cell based assays as exemplified in Example 3. Preferably, the T1R-fusion proteins comprise a functional G protein, e.g., Galpha-15 or chimeric G protein like the ones previously identified, or another G protein that is capable of coupling the chimeric receptor to an intracellular signaling pathway or to a signaling protein such as phospholipase C. (Offermanns, 2003; Offermanns and Simon, 1995; Ueda et al., 2003) For the development of such cell based assays the fusion proteins will be subcloned into the multicistronic expression vector backbone leading to expression vector of the type pTrix-Eb-R2Gq-R3 depicted in FIG. 4.

The figures show:

FIG. 1: Bioinformatic alignment with Clustal X software: Different GPCR nucleotide sequences of the T1R class are presented in an alignment analysis. Human wild type cDNA sequences are depicted as wt_hT1R5; the theoretically most optimized cDNA sequences after multi parameter optimization are depicted as opt_hT1R5; partly optimized cDNAs used for stable cell line development are termed sh_T1Rs. The nucleotide alignments are presented as phylogenetic trees in which wt_T1R1 was defined as the outgroup. The underlying distance matrix is shown below.

FIG. 2: Multicistronic eucaryotic expression vector pTrix-Eb-R2R3: The expression of the taste receptor genes shT1R2, shT1R3 and blasticidin S deaminase (bsd) gene are under the control of the human elongation factor 1 alpha promoter (PEF1a). To confer multicistronic expression on the translational level two internal ribosomal entry sites (Cite I and Cite II) have been inserted. The multicistronic unit is terminated by a simian virus 40 polyadenylation site. The prokaryotic origin of replication (ori) and the kanamycin resistance gene serve for the propagation, amplification and selection of the plasmid vector in E. coli.

FIG. 3: The stable HEK_Ga15#17R2R3b #8 was selected with blasticidin and G418 after transfection of the stable HEK_Ga15#17 (G418) with pTriX_Eb_R2R3. This tricistronic expression was necessary for the selection and retrieval of stable clones. Clone #8 shows calcium responses to several artificial sweeteners and a natural sweet protein.

FIG. 4: Multicistronic eucaryotic expression vector pTrix-Eb-R2R3: The expression of the taste receptor genes shT1R2-Gq, shT1R3 and blasticidin S deaminase (bsd) gene are under the control of the human elongation factor 1 alpha promoter (PEF1a). To confer multicistronic expression on the translational level two internal ribosomal entry sites (Cite I and Cite II) have been inserted. The multicistronic unit is terminated by a simian virus 40 polyadenylation site. The prokaryotic origin of replication (ori) and the kanamycin resistance gene serve for the propagation, amplification and selection of the plasmid vector in E. coli.

Experimental Materials and Methods Cell Culture

Transient transfection/selection of stable HEK293 cells—Transient and stable transfections can be performed with lipid complexes like calcium phosphate precipitation, Lipofectamine/PLUS reagent (Invitrogen), Lipofectamine 2000 (Invitrogen) or MIRUS TransIT293 (Mirus Bio Corporation) according to the manuals. Electroporation can also be a method of choice for stable transfection of eukaryotic cells.

The cells are seeded in 6-well plates at a density of 4×105 cells/well. HEK293 cells are transfected with linearised plasmids for stable expression of the genes of interest. After 24 hours, the selection with selecting reagents like zeocin, hygromycin, neomycin or blasticidin starts. About 50 μl to 300 μl trypsinized transfected cells from a 6-well are seeded in a 100 mm dish and the necessary antibiotic is added in an appropriate concentration. Cells are cultivated until clones are visible on the 100 mm cell culture plate. These clones are selected for further cultivation and calcium imaging. It takes about four to eight weeks to select cell clones which stably express the genes of interest.

Calcium Imaging

Fluo-4 AM assay with stable HEK293 cells—Stable cells are maintained in DMEM high-glucose medium (Invitrogen) supplemented with 10% fetal bovine serum (Biochrom) and 4 mM L-glutamine (Invitrogen). Cells for calcium imaging are maintained in DMEM low-glucose medium supplemented with 10% FBS and 1× Glutamax-1 (Invitrogen) for 48 hours before seeding. These stable cells are trypsinized after 48 hours (either with Trypsin-EDTA, Accutase or TrypLE) and seeded onto poly-D-lysine coated 96-well assay plates (Corning) at a density of 45,000 cells/well in DMEM low-glucose medium supplemented with 10% FBS and 1× Glutamax-1.

After 24 hours, the cells were loaded in 100 μl medium with additional 100 μl of 4 μM Fluo-4 (calcium sensing dye, 2 μM end concentration; Molecular Probes) in Krebs-HEPES (KH)-buffer for 1 hour. The loading reagent is then replaced by 80 μl KH-buffer per well. The Krebs-HEPES-buffer is a physiological saline solution including 1.2 mM CaCl2, 4.2 mM NaHCO₃ and 10 mM HEPES.

The dye-loaded stable cells in plates were placed into a fluorescence microtiter plate reader to monitor fluorescence (excitation 488 nm, emission 520 nm) change after the addition of 20 μl KH-buffer supplemented with 5× tastants. For each trace, tastant was added 11.5 seconds after the start of the scan and mixed two times with the buffer, scanning continued for an additional 32 seconds, and data were collected every second.

Data analysis/Data recording—Calcium mobilization was quantified as the change of peak fluorescence (AF) over the baseline level (F). Data were expressed as the mean S.E. of the AF/F value of replicated independent samples. The analysis was done with the software of the microtiter plate reader.

EXAMPLES

The following examples are provided to illustrate preferred embodiments and are intended to be illustrative and not limitative of the scope of the invention. In the DNA sequences presented herein, the one letter codes N or n refers to any of the of the four common nucleotide bases, A, T, C, or G. In the protein sequences presented herein, the one-letter code X or Xaa refers to any of the twenty common amino acid residues.

Example 1 Synthesis and Design of Synthetic Intronless hT1R2 and hT1R3 cDNAs

The nucleotide sequence of the human receptors hT1R2 and hT1R3 are based on their wild type coding DNAs and have been optimized according to a multiparameter analysis considering optimal codon usage, putative cryptic splice sites, putative repeated sequences as well as AT-rich or GC-rich sequence stretches. Gene optimization often has favorable effects on enhanced mRNA stability, translational efficiency and reduced RNA secondary structure to prevent transcriptional pausing or premature termination of transcriptional elongation.

Up to now the binding sites for only a few ligands of the sweet taste heterodimeric receptor T1R2/T1R3 are known. Hence, the encoded wild-type amino acid sequences of the receptors were left unchanged in this multiparameter optimization, to retain the receptors binding qualities.

Synthesis and construction of the receptor cDNAs was done via the assembly of synthetic oligonucleotides and subsequent cloning into a standard pUC-18 plasmid vector for further amplification. Alternatively, these nucleic acids can be cloned in vitro by well-known cloning techniques., (Ausubel et al., 1998; Pachuk et al., 2000; Sambrook et al., 1989; Stemmer et al., 1995). Double-stranded DNA fragments may then be obtained either by synthesizing the complementary strand and annealing the strands together under appropriate conditions, or by adding the complementary strand using DNA polymerase with an appropriate primer sequence.

Due to the synthetic generation of the cDNAs the receptors of the present invention have been described with the prefix “s” and termed shT1R2 and shT1R3. The nucleic acid and amino acid sequences for the above mentioned T1R cloned sequences as well as other full-length and partial T1R sequences are set forth in the sequence protocol:

Example 2 Multicistronic Vectors for the Expression of Taste Receptors

For the construction of a multicistronic expression unit the taste receptor sequences displayed in Example 1 have been used. As shown in FIG. 2 the tricistronic expression unit is under the control of the human elongation factor 1 alpha promoter. Using standard cloning techniques the cDNA for the receptors shT1R2 and shT1R3 and the cDNA for the blasticidin S deaminase gene have been cloned. To enable the translation initiation of each gene of this tricistronic unit two EMC-virus derived internal ribosomal entry sites (IRES—also termed Cap-independent translation enhancer (CITE)) have been inserted. (Jackson et al., 1990; Jang et al., 1988) The tricistronic expression unit is terminated by a simian virus 40 polyadenylation signal sequence. This composition permits the simultaneous expression of all three genes under the control of only one promoter. In contrast to monocistronic transcription units, which integrate independently from each other into different chromosomal locations during the process of stable cell line development, the tricistronic transcription unit integrates all containing genes in one and the same chromosomal locus. Due to the alignment of the genes, the blasticidin S deaminase gene is only transcribed in case a full length transcription takes place. Moreover the polarity of multicistronic transcription units (Moser et al., 2000) leads probably to a balanced stoichiometry of the receptor genes and their expression rates in the range of 1:0.7 up to 1:1 for the first two positions whereas the blasticidin S deaminase gene compared to the receptor genes in the third position is expressed to a lesser extend. Assuming that for the functional heterodimeric receptor shT1R2/shT1R3 a 1:1 stoichiometry is needed the lesser polarity effects for the receptor genes promote the desired stoichiometry whereas the reduced expression of the deaminase promotes an integration locus with enhanced transcriptional activity.

Example 3 Detection of T1R2/T1R3 Dependent Activity

In wild type taste cells—e.g. in the human taste bud—signal transduction is presumably transduced by the G-proteins gustducin and/or by G-Proteins of the Galpha-i type. Encountering sweet ligands the heterodimeric taste receptor T1R2/T1R3 reacts with induction of second messenger molecules; either induction of the cAMP level in response to most sugars or induction of the calcium level in response to most artificial sweeteners. (Margolskee, 2002)

To analyze the function and activity of the heterodimeric T1R2/T1R3 taste receptor a calcium dependent cell based assay has been utilized. Briefly, synthetic T1R type taste receptors (as shown in Example 1) have been transfected with the plasmid vector pTrix-Eb-R2R3 (see Example 2) in a HEK293 cell line stably expressing the mouse G-alpha-15 G-protein. Selection of T1R2/T1R3 expressing cells has been performed by culturing the transfected cells in the presence of blasticidine.

For measurement of T1R2/T1R3 taste receptor dependent activity HEK293 cells stably expressing G-alpha-15, shT1R2 and shT1R3 were 4×104 seeded in 96-well plates and labeled with the calcium sensitive fluorescence dye Fluo4-AM (2 μM) in DMEM culture medium for one hour at 37° C. For the measurement in a fluorescence plate reader the medium was exchanged for KH-buffer and incubated for another 15 minutes at 37° C. Fluorescence measurement of the labeled cells was conducted in a Novostar fluorescence plate reader (BMG, Offenburg, Germany). Response to different tastants as depicted in FIG. 3 was recorded as Fluo4-AM fluorescence increase initiated through the T1R2/T1R3 dependent increase of the second messenger calcium. After obtaining calcium signals for each sample, calcium mobilization in response to tastants was quantified as the relative change (peak fluorescence F1−baseline fluorescence F0 level, denoted as dF) from its own baseline fluorescence level (denoted as F0). Though rel. RFU is dF/F0. Peak fluorescence intensity occurred about 20-30 sec after addition of tastants. The data shown were obtained from two independent experiments and done in triplicates.

REFERENCES

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1. A method for the identification of a compound that potentiates the sweetness of a sweet tasting compound, comprising the steps of: a. providing a transformed eukaryotic host cell; wherein said host cell is transformed with genetic sequences coding for at least two different G-protein coupled receptors (GPCRs), wherein the transformation was accomplished with a multicistronic expression vector which comprises a multicistronic expression unit comprising downstream from a promoter for the expression in an eukaryotic host cell and functionally linked thereto, the following cistrons: I. GPCR₁ II. GPCR₂, and III. a selection marker, wherein the GPCR₁ and the GPCR₂ are each independently selected from the group consisting of T1R or T2R taste receptors, b. cultivating the transformed host cell under conditions sufficient for the functional expression of said at least two different GPCRs, c. contacting the cultivated host cell expressing the at least two different GPCRs in a functional manner with a sweet tasting compound and a candidate compound that potentially potentiates the sweetness of a sweet tasting compound, d. measuring intracellular calcium levels of the transformed host cell on exposure to the sweet tasting compound and the candidate compound, and e. selecting a compound that increases the intracellular calcium levels in the presence of the sweet compound greater than the sweet tasting compound alone, wherein the greater increase in the intracellular calcium levels is indicative of a compound that potentiates the sweetness of a sweet tasting compound.
 2. A method according to claim 1, wherein the host cell is selected from the group consisting of: HEK293 (human embryo kidney); Hela (Human Negroid cervix epitheloid carcinoma); HT29 (Human Caucasian colon adenocarcinoma grade IQ); A431 (human squamous carcinoma); IMR 32 (human caucasian neuroblastoma); K562 (Human Caucasian chronic myelogenous leukaemia); U937 (Human Caucasian histiocytic lymphoma); MDA-MB-231 (Human Caucasian breast adenocarcinoma); SK-N-BE(2) (Human Caucasian neuroblastoma); SH-SY5Y (Human neuroblastoma); HL60 (human promyelocytic leukemia); CHO-KI (Hamster Chinese ovary); COS-7 (Monkey African green kidney, SV40 transformed); S49 (mouse lymphoma); Ltk (Mouse C34/An connective tissue); NG108-15 (Mouse neuroblastoma×Rat glioma hybrid); B50 (Rat nervous tissue neuronal, ECACC); C6 (Rat glial tumour); Jurkat (Human leukaemic T cell lymphoblast); BHK (Hamster Syrian kidney); and Neuro-2a (Mouse Albino neuroblastoma); and NIH/3T3 (mouse embryo fibroblast).
 3. A method according to claim 1, wherein the selection marker is selected from the group consisting of hygromycin, zeocin, neomycin, blasticidin^(r) and puromycin^(r).
 4. The method of claim 1, wherein both the GPCR₁, the GPCR₂, and the selection marker are functionally connected by intervening Internal Ribosome Entry Sites (IRES) selected from the group consisting of IRES_(EMCV), derived from encephalomyocarditis virus (synonym: CITE_(EMCV)); IRES_(GTX), derived from the GTX homeodomain mRNA; IRES_(BM3), derived from cold-inducible Rbm3; IRES_(PV), derived of polioviral origin; IRES_(RV), derived from rhinovirus; IRES_(FMDV), derived from foot and mouth disease virus; IRE_(HV), derived from hepatitis C virus; IRES_(CSFV), derived from classic swine fever virus; IRES_(BVDV), derived from bovine viral diarrhea virus; IRES_(FMLV), derived from friend murine leukemia virus gag tnRNA; IRES_(MMLV), derived from moloney murine leukemia virus gag mRNA; IRESmv, derived from human immunodeficiency virus env mRNA; IRES_(PSIV), derived from Plautia stali intestine virus; TRES_(RPV), derived from Rhopalosiphum padi virus; and IRES_(KSH), derived from Karposi's sarcoma-associated herpesvirus.
 5. The method of claim 1, wherein the multicistronic expression unit is terminated by a polyadenylation signal.
 6. A method according to claim 1, wherein the multicistronic expression vector additionally comprises a cistron coding for a G-protein.
 7. A method according to claim 1, wherein the candidate compound is selected from the group consisting of small molecules and peptides.
 8. The method of claim 1, wherein the sweet tasting compound is selected from the group consisting of glucose, fructose, saccharose, acesulfam K, saccharin, cyclamat, aspartam, xylitol, stevioside, sucralose, thaumatin, moncllin, brazzein, perillartine, glycyrrhizin, sucronic acid, P-4000, SC45647, NC174, neohesperidin, and sweet tasting amino acids.
 9. A method according to claim 1, wherein GPCR₁ is human T1R2 and wherein GPCR₂ is human T1R3.
 10. A method according to claim 1, wherein the promoter is selected from the group consisting of: cytomegalovirus promoter (P-CMV), human elongation factor 1 alpha promoter (P-EF1alpha), human ubiquitin promoter (P-ubi), simian virus promoter (P-SV40), and Rous sarcoma virus long terminal repeat promoter (P-RSV-LTR).
 11. The method according to claim 1, wherein the multicistronic expression vector additionally comprises a cistron coding for a G-protein.
 12. The method of claim 11, wherein the G-protein is located between the last GPCR and the selection marker and is functionally connected to both via an Internal Ribosome Entry Site (IRES).
 13. The method of claim 11, wherein the cistron codes for G-alpha
 15. 14. The method of claim 1, further comprising transforming said eukaryotic host cell with a multicistronic expression vector which comprises a multicistronic expression unit comprising downstream from a promoter for the expression in an eukaryotic host cell and functionally linked thereto, the following cistrons: I. GPCR₁ II. GPCR₂, and III. a selection marker, wherein the GPCR₁ and the GPCR₂ are each independently selected from the group consisting of T1R or T2R taste receptors.
 15. The method of claim 1, further comprising contacting the cultivated host cell expressing the at least two different GPCRs in a functional manner with a sweet tasting compound alone; measuring intracellular calcium levels of the transformed host cell on exposure to the sweet tasting compound alone; and selecting a compound which induces a greater change in the intracellular calcium levels in the presence of the sweet tasting compound than the sweet tasting compound alone.
 16. The method of claim 1, further comprising contacting the cultivated host cell expressing at least two different GPCRs in a functional manner with the candidate compound alone, measuring intracellular calcium levels of the transformed host cell on exposure to the candidate compound alone, and selecting a compound which does not induce a change in the intracellular calcium levels when administered alone. 